1. Field of the Invention
The present invention relates to a controller for a drive system, and in particular to a technique of charging a battery and a capacitor connected in parallel with the battery with electric power generated during regenerative braking of a vehicle.
2. Description of the Background Art
In recent years, as part of measures to address environmental problems, a hybrid vehicle that runs by driving force obtained from at least one of an engine and a motor has received attention. A battery, a capacitor (condenser) and the like are mounted on such a hybrid vehicle for storing electric power to be supplied to the motor.
Japanese Patent Laying-Open No. 11-332012 discloses a vehicle drive system having an engine and an assist motor. The vehicle drive system described in Japanese Patent Laying-Open No. 11-332012 includes an engine generating main propulsion for the vehicle, an assist motor generating propulsion for assisting the vehicle, a starter motor driven by electric power supplied from an auxiliary power storage portion (auxiliary battery) to start the engine, an electric power converter boosting the voltage of the electric power supplied from the auxiliary power storage portion to be a voltage required for the assist motor, a power storage portion (capacitor) for the motor, the power storage portion temporarily storing the electric power whose voltage is boosted by the electric power converter and supplying the stored electric power to the assist motor, and a supplemental charge control portion controlling and regeneratively driving the electric power converter, and using the electric power stored in the power storage portion for the motor to supplementally charge the auxiliary power storage portion. The starter motor is driven by the electric power supplementally stored in the auxiliary power storage portion to start the engine.
According to the vehicle drive system described in the document, electric energy left in the power storage portion for the motor at the last time when the drive system went down is used for starting the engine. By regeneratively driving the electric power converter, the electric power left in the power storage portion for the motor is used to supplementally charge the auxiliary power storage portion. An amount of electric power stored in the auxiliary power storage portion is thereby increased, which allows the auxiliary power storage portion to supply sufficient electric power to the starter motor so that startability of the engine can be improved.
In the vehicle drive system described in Japanese Patent Laying-Open No. 11-332012, electric power is stored in the power storage portion (capacitor) for the motor. At this time, a voltage of the capacitor is generally controlled such that the voltage does not exceed a voltage which is such that the system is considered to be malfunctional when the voltage is reached (e.g. rated voltage). However, even if the voltage of the capacitor is transiently increased because of an operational delay of a charge control (charge control device) and the like, the system is considered to be malfunctional, which may prohibit the vehicle from running.